Binocular head mounted display systems allow a user to view an image or images of displayed information with both eyes. Typical binocular head mounted display systems include two displays, one for each of the user's eyes, as well as two sets of identical optics so as to be very costly and heavy. Binocular head mounted display systems that include only a single display are also known. Both types of systems typically direct the user's eyes such that the axes of the eyes are parallel for viewing a projected image focused at infinity. For virtual reality applications in which the user is to be totally immersed in the video image, these binocular systems are further formed so that the user's field of view is completely blocked, the user being unable to see anything but the displayed image.
These known systems have a number of problems. For example, it has been found that with head mounted display systems in which the user cannot focus on anything but the projected image, eye fatigue often occurs. Such systems can cause feelings of claustrophobia because the user's view is completely enclosed and feelings of anxiety because the user can hear things going on in his surroundings but is not able to see what is happening. With these systems the user can also experience "sea sickness" when the motion of the image that he is viewing does not coincide with the motion that the user's body is experiencing.
Binocular systems in which the user's eyes are directed such that the axes of the eyes are parallel have been found to contribute to eye fatigue when the image is focused at less than infinity. Eye fatigue and feelings of discomfort result because it is unnatural for a person's eyes to be such that the axes of the eyes are parallel when viewing an image that is only two or three feet away.
Known binocular systems are typically designed for users having a particular "average" or "normal" interpupillary distance (hereinafter referred to as IPD). Because the IPD of users can vary considerably, these systems are usable only by a very limited portion of the population and are not suitable for general consumer applications. If a user has an IPD that varies considerably from the average IPD for which the system is designed his eye will be off of the axis of the optical system. User's who are off-axis typically see right eye and left eye images that do not line up. They may also see distortions in the image wherein one side of the image will look bigger than the other side, and/or a part of the image will be in focus while other parts of the image will be out of focus. Further, a portion of the image depicted on the display may be cut off when viewed off axis through the optics of the system.